Ligand-Modulated Chemical and Structural Implications in Four‑, Five‑, and Six-fold Coordinated Aluminum Heteroaryl Alkenolates

Synthesis
and characterization (gas phase, solution, and solid-state)
of a series of four-, five- and six-fold coordinated heteroaryl-alkenolato
aluminum complexes were performed to demonstrate the delicate interplay
of structural and chemical influences of ligands in the design of
new precursors for chemical vapor deposition. We are investigating
the properties of heteroaryl alkenols as O^N chelating ligands [where
O^N is 3,3,3-trifluoro­(pyridin-2-yl)­propen-2-ol (H-PyTFP), 3,3,3-trifluoro­(1,3-benzimidazol-2-yl)­propen-2-ol
(H-BITFP), 3,3,3-trifluoro­(dimethyl-1,3-oxazol-2-yl)­propen-2-ol (H-DMOTFP),
3,3,3-trifluoro­(1,3-benzoxazol-2-yl)­propen-2-ol (H-BOTFP), 3,3,3-trifluoro­(1,3-benzthiazol-2-yl)­propen-2-ol
(H-BTTFP), and 3,3,3-trifluoro­(dimethyl-1,3-thiazol-2-yl)­propen-2-ol
(H-DMTTFP)] to prepare volatile and air-stable compounds. All three
methyl groups in highly reactive AlMe3 could be replaced
by H-PyTFP, H-BITFP, H-DMOTFP, and H-BOTFP yielding octahedral complexes
of the type Al­(O^N)3; under similar conditions H-BTTFP
and H-DMTTFP produced heteroleptic MeAl­(O^N)2 compounds
with five-fold coordinated aluminum centers. Various attempts to obtain
tris-alkenolato derivatives by choosing higher temperatures and prolonged
reaction times were not successful. The reaction of H-PyTFP with [Al­(OtBu)3]2 produced the
dimeric heteroleptic [Al­(PyTFP)­(OtBu)2]2 complex with Al atoms present in both octahedral
(Oh) and tetrahedral
(Td) coordination in
a single molecular unit. The introduction of the chelating ligand
H-PyTFP in the dimeric framework of [Al­(OtBu)3]2 enhanced the stability against hydrolyses
significantly. The tendency of Al­(III) centers to preferably coordinate
in Td or Oh environment was elucidated by hydrolysis
studies of monomeric Al­(PyTFP)3, Al­(BOTFP)3,
and MeAl­(BTTFP)2 that produced hydroxo-bridged dimers to
retain the octahedral environment for Al atoms. Surprisingly, hydrolysis
of monomeric MeAl­(DMTTFP)2 yielded an oxo-bridged dimer
with two five-fold coordinated aluminum centers. The structural features
of all new complexes were investigated in solution, vapor, and solid
state by multinuclear NMR spectroscopy, EI-MS spectrometry, and single-crystal
X-ray diffraction analyses, respectively.